A typical active matrix display panel system, of various display technologies such as LCD, ePaper, and electrophoretic display, is shown in FIG. 1. Referring to FIG. 1, source lines SO1, SO2, . . . , SOm−1, SOm are driven by source driving circuits 101. Gate lines GO1, GO2, . . . , GOn−1, GOn are driven by gate driving circuits 103. A common electrode (VCOM) 105 of all pixels are connected, and is driven by VCOM driving circuits 107. A timing controller 109 provides timing control signals for the source driving circuits 101, the gate driving circuits 103 and the VCOM driving circuits 107. A power generator 111 provides DC power for the above mentioned circuits. For example, DC power of VS1 and VS2 are provided by the power generator 111 to the source driving circuits 101, which outputs voltage levels of Vs1 and Vs2 to source lines.
Referring to FIG. 1, the VCOM driving circuits 107 include a VCOM Driver, which is a voltage driving circuit with its output connected to the VCOM electrode 105 of the display panel. One large stabilizing capacitor 113 is connected between the VCOM electrode 105 and the ground. This capacitor is configured to reduce the noise on the VCOM electrode 105 during the display period. The display panel can be modeled as a capacitor connected between the VCOM electrode 105 and the ground.
FIG. 2 is schematic diagram of one display pixel of the display panel depicted in FIG. 1. Referring to FIG. 2, the display pixel includes a switch element 201, such as a thin film transistor (TFT); a storage capacitor Cst; a pixel display element, modelled by a capacitor Clc; and parasitic capacitance, modelled by a capacitor Cgs. The gate and drain electrodes of the TFT 201 are connected to one gate line GOi, and one source line SOj of the display panel respectively. The source electrode of the TFT 201 is connected to the Clc and the Cst. The other terminal of Clc and Cst are connected to a VCOM electrode of display panel.
There are two conventional methods for driving display panels: the DC-VCOM method and the AC-VCOM method. The resultant voltages across 3 terminals (GOi, SOj, VCOM) of a pixel are the same in both method, which conform to the panel driving requirement. With the DC-VCOM Method, the VCOM voltage remains at a constant level of Vcomc, so is the voltage across the stabilizing capacitor 113 (as shown in FIG. 1). With the AC-VCOM Method, the VCOM voltage alternates, so that the driving voltage levels of source and gate voltages can be reduced. Alternatively, instead of reducing driving voltage levels, the resultant pixel voltages can be increased without increasing the driving voltage levels. With this method, the VCOM driver keeps charging and discharging the stabilizing capacitor 113, thereby consuming a considerable amount of power. FIG. 3 shows a waveform of VCOM voltage in the AC-VCOM method. Referring to FIG. 3, as the VCOM voltage alternates between Vcomc (−2V), Vcom1 (13V) and Vcom2 (−17V), the voltage across the stabilizing capacitor 113 (as shown in FIG. 1) alternates between −2V, 13V and −17V. The voltage variation across the stabilizing capacitor is relatively large. The voltage and capacitance figures are illustrative. Different display panels may have different driving voltage level requirements, and feature different capacitance characteristics.